The present invention relates to a sewing machine for forming running-stitch seams.
Sewing machines for forming a seam with a so-called running stitch are known; such seam is formed with a single thread of preset length, known as draw, and is composed of stitches that are alternately visible on either side of the fabric being sewn.
A machine of this kind is disclosed in U.S. Pat. No. 4,122,787 and is substantially composed of a needle that has a substantially vertical axis and is provided, proximate to its tip, with an open eye and is orientated so that its axis is substantially perpendicular to a supporting surface on which the item being sewn is deposited.
The needle can be actuated with a reciprocating motion along its axis in order to pass cyclically through the item deposited on the supporting surface. Below the supporting surface there is a thread take-up wheel, which is arranged on a plane that is substantially parallel to the axis of the needle and is orientated so that its axis is substantially perpendicular to the axis of the needle. Such wheel is composed of a fixed part, in which there is a magazine for the taken-up thread, and a rotatable part, which can be actuated with a rotary motion about its own axis. The rotatable part of the wheel is mounted coaxially on an actuation shaft, which can be rotationally actuated about its own axis synchronously with the movement of the needle, and has, in a peripheral region, a looper or lower looper which, as a consequence of the rotation of the rotatable part of the wheel and of the movement of the needle, engages the thread carried below the supporting surface by the needle and takes it up, making it pass inside the magazine provided in the fixed part of the wheel.
The machine is completed by thread handling elements, which are located below the supporting surface, and by another looper, or upper looper, which is arranged above the supporting surface laterally to the needle and is adapted to engage the loop of thread carried by the needle above the supporting surface and retain it while it is taken up by the lower looper.
Substantially, this machine works with a draw of thread that is clamped at one of its ends. During a first step of the operating cycle, the needle, threaded with the thread proximate to its clamped end, passes through the item being sewn, carrying a loop of thread below the supporting surface.
Said loop of thread is engaged by the lower looper as a consequence of the rotation of the rotatable part of the wheel, and all the excess thread of the draw is loaded into the magazine located in the fixed part of the wheel and disengaged from the needle, which is extracted above the supporting surface.
After the item being sewn has been advanced by one step, the needle descends again with its eye below the supporting surface and engages the thread, carrying in the subsequent return step a loop of thread above the item being sewn. This loop of thread is engaged by the upper looper and, after the item has been advanced by another step, the needle again passes through the item, carrying a loop of thread below the supporting surface. Said loop of thread is engaged by the lower looper and disengaged from the needle. The lower looper, by continuing its rotation, takes up the portion of thread that lies between the eye of the needle and the free end of the draw, extracting it and reloading it into the magazine of the fixed part of the wheel. In this manner, the machine produces a seam with a single thread composed of stitches that are alternatively visible on the outer side and inner side of the item being sewn, i.e. a seam formed with so-called running stitches.
Over time, this machine has proved to be susceptible of improvements aimed mainly at improving the quality and precision of the seam.
The aim of the present invention is to provide a sewing machine for forming running-stitch seams that ensures highly regular execution of the stitches.
Within this aim, an object of the invention is to provide a sewing machine for forming running-stitch seams that ensures a substantially uniform tension of the thread during the forming of the stitches.
Another object of the invention is to provide a sewing machine for forming running-stitch seams that can use for the seam almost the entire length of the draw of thread, thus allowing to save both time and material.
Another object of the invention is to provide a sewing machine that can be obtained, with modifications that are relatively simple to provide and have a modest cost, from known types of sewing machine for forming running-stitch seams.
This aim and these and other objects that will become better apparent hereinafter are achieved by a sewing machine for forming running-stitch seams, comprising a needle provided, proximate to its tip, with an open eye and orientated so that its axis is substantially perpendicular to a surface for supporting the item being sewn, said needle being actuatable with a reciprocating motion along its own axis in order to pass cyclically through said supporting surface and the item being sewn; a thread take-up wheel being provided below said supporting surface, being arranged on a plane that is substantially parallel to the axis of said needle and being orientated so that its axis is substantially perpendicular to the axis of said needle; said wheel being composed of a fixed part, in which there is a magazine for the taken-up thread, and a rotatable part, which can be rotationally actuated about its own axis; said rotatable part of the wheel being mounted coaxially on an actuation shaft that can be rotationally actuated about its own shaft axis synchronously with the movement of said needle and being provided with a lower looper that can engage the loop of thread carried below said supporting surface by said needle in order to load the thread in said magazine formed in the fixed part of the thread take-up wheel; characterized in that it comprises tensioning means for the pneumatic tensioning of the portion of thread that lies below said supporting surface between the item being sewn and said taken-up thread magazine.